Computers utilize a variety of data storage approaches for mass data storage. Various types of data storage devices and organization of groups of data storage devices are used to provide primary storage, near line storage, backup storage, hierarchical storage, and various types of storage virtualization and data replication.
Data storage devices include tape storage, disk drives, optical drives, and solid state disks. In terms of performance, solid state disks provide the best performance, followed by hard disk drives. Optical and tape storage devices provide significantly slower performance compared to hard disk drives and solid state disks.
Within a given storage device type, various storage devices may have different performance attributes. For example, hard disk drives come in multiple rotation speeds, cache sizes, track density, and other physical parameters. Rotation speeds of 5,400, 7,200, 10,000, and 15,000 RPM are currently available, with cache sizes ranging from 32 MB (Megabytes) to 8 GB (Gigabytes) and more. Therefore, it is possible to create sub-groups of a particular storage device type based on performance attributes of each sub-group.
Although it would be desirable to have unlimited amounts of the fastest possible data storage, in most cases that approach is cost prohibitive and a waste of money. Solid state disks, for example, make a very inefficient choice for offline data storage, where data can often be written off-hours when data networks and servers are lightly used, and the content is rarely accessed. Additionally, data storage needs almost always increase over time in order to accommodate new data to be stored, backed up, virtualized, and so on.